Metal shathed carbon electrode



Feb. 7, 1967 M. B. DELL 3,303,119

METAL SHEATHED CARBON ELECTRODE Filed June 25, 1962 ALUMINUM FOIL H I MWMi "WW2 2 ANODE CARBON ANODE CARBON MAST/c INVENTOR.

M. BENJA MIN DELL United States Patent 3,303,119 METAL SHEATHED CARBONELECTRODE Manuel Benjamin Dell, Pittsburgh, Pa, assignor to AluminumCompany of America, Pittsburgh, Pa., 21 corporation of PennsylvaniaFiled June 25, 1962, Ser. No. 210,284 5 Claims. ((11. 204-290) Thisapplication is a continuation-in-part of my similarly entitledapplication Serial No. 193,683, filed May 10, 1962, now abandoned.

This invention relates to metal sheathed carbon, especially foilsheathed carbon anodes for use in an electrolytic cell for the roductionof aluminum, and to mastic compositions especially adapted for adheringmetal foil to baked carbon. More specifically, this invention relates tomastic compositions useful for foil sheathing of baked carbon anodes toprotect them against severe air burning when subsequently, duringoperation of a cell of the above type, they are exposed to andmaintained at elevated temperatures gradually approaching the meltingpoint of the aluminum and finally exceeding that temperature.

In the smelting of aluminum by electrochemical decomposition of aluminadissolved in a molten electrolyte, a conventional electrolytic cellcomprises, in general, a steel shell provided with a carbon liningforming the cell cavity and serving as the cathode. Insulating materialis generally used between the carbon lining and the shell. In oneconventional arrangement, current carrying bus bars are supported abovethe cavity of the cell, and one or more carbon anodes hang from thesebus bars and dip into the electrolyte.

In operation, a mixture of alumina and cryolite (usually with one ormore other fluorides) is provided in the cell cavity, and an electriccurrent is passed through the cell. The resistance of thealumina-cryolite charge to the passage of current produces sufficientheat to fuse the same, and form a molten electrolyte or bath, which maythen be considered as a solution of alumina in molten cryolite. Aluminumis electrolyzed from the solution, depositing as a molten layer on thecathode, while oxygen passes to the anode. A crust of frozen electrolyteforms on the surface of the bath (which is usually at a temperature ofabout 970 C.) and this crust is usually covered over with someundissolved alumina.

The anode carbon is exposed to elevated temperatures on the order of 400to 500 C. near its top and to electrolyte temperature at the bottom. Theoxygen passing to the anode carbon reacts with the hot anode carbon toform carbon dioxide (which to some extent is subsequently reduced tocarbon monoxide by the hot carbon). Operating data confirm thatapproximately 0.4 pound of carbon per pound of aluminum metal producedis necessarily consumed in this manner. Accommodation for this loss ismade by lowering periodically anodes (taller than are required at theoutset of operations) usually with only a portion of the anode at anytime being submerged in the electrolyte. As the anode carbon isconsumed, the anode is lowered into the bath by mechanical or automaticmeans.

The bonnet and upper side surfaces of the anode, referred to hereinafteras simply the head, protrude above the electrolytic bath during at leastthe initial period of operation. The anode carbon becomes heated asdescribed, and the head (being exposed to the oxygen of the air) issubject to oxidation, this action being generally referred to as airburning. This adds substantially to the carbon consumption. The averagenet anode carbon consumption, which of course is somewhat dependent onthe size, shape and quality of the anode carbon will as ice a generalrule be not less than 0.5 pound of carbon per pound of aluminum metalproduced (approximately 0.1 pound greater than that directly consumed byoxygen coming from electrolysis). Net anode carbon consumption, as usedherein, refers to the total pounds of baked anode carbon introduced intothe cell less the weight of the unconsumed anode carbon butt removedfrom the cell when replacing anodes.

To reduce air burning, the conventional practice is to cover the head ofthe anode carbon with a blanket of alumina and solidified electrolytemixed therewith generally referred to as an alumina blanket (or oreblanket). The depth of such a blanket must obviously be somewhatlimited. In order to permit covering the side walls of the anode as wellas possible, the height of the anode employed in the cell is usuallyrestricted below that which might otherwise be used. In other words, theupper side Walls of very tall anodes cannot be afforded desiredprotection against air burning by means of an alumina blanket. Moreover,the alumina blanket is somewhat permeable in any case and air thereforereaches the anode carbon through it, with some inevitable air burning ofthe anode carbon so protected.

The power input to the cell usually exceeds that required for theelectrochemical decomposition of the alumina, and maintains theelectrolyte molten. Under steady operating conditions, excess heat isallowed to dissipate from the cell to maintain optimum operatingtemperature. Since the conventional alumina blanket acts as aninsulatingmedium, some heat which might escape from the cell by radiation from theanode is partially restrained from doing so. This limits metalproduction from the cell, since current input must be reducedaccordingly, and this means that the amount of aluminum metal producedis proportionally reduced. It is desirable, however, to increase thecurrent, to produce a corresponding increase in metal production, if theadditional heat can be dissipated as by eliminating or reducing thealumina blanket.

While efforts have been made in the past to protect anode carbons fromair burning by aluminum sheaths, it has been found that such sheaths arewholly inadequate unless intimately and tightly adhered to the bakedcarbon surface. Adhesives previously thought to be most suitable (andinexpensive) for adhering aluminum foil to anode-carbons are of theinorganic type, but such adhesives have the disadvantage thatcontaminants are introduced into the cell, i.e. the molten bath, theanode carbon butts (which are reclaimed), and the aluminum produced.

It is therefore a general object of this invention to provide a masticadhesive composition for adhering aluminum foil to baked carbon whichwill withstand elevated temperatures and which, when used in connectionwith anode carbon for aluminum production, will not introduce inorganiccontaminants.

Another general object of this invention is to provide a baked carbonanode, for use in an electrolytic cell for the production of aluminum,having an improved sheathing for substantially reducing anode carbonconsumption resulting from air burning.

It is an advantage of the invention that its use obviates the need formaintaining an alumina blanket over the head of the anode, therebyrendering economical the use of taller carbon anodes and furtherpermitting increased current input to the cell without modifyingconventional cell design or installed facilities.

The mastic composition of this invention is one especially adapted foradhering aluminum foil to baked anode carbon subsequently to be exposedto and maintained at elevated temperatures approaching the melting pointof the aluminum. It should contain, as the essential binder component,about 40 to 50 percent by weight, preferably 42 to 47 percent,bituminous tar or pitch or mixtures thereof. Such binder material shouldhave a softening point less than about 110 C., conveniently about 40 C.Aromatic bituminous materials, of the characteristic just mentioned suchas crude or refined coal tar, coal tar pitch, petroleum pitch ormixtures thereof, may be used most advantageously.

As the essential aggregate component, the mastic composition shouldcontain about 60 to 50 percent by weight, preferably 58 to 53 percentcoal, coke or mixtures thereof. This aggregate material should be finelydivided; the bulk of it should pass a 100 mesh screen, preferably a 200mesh screen. Some larger particles are not objectionable, butsubstantially all of the aggregate should pass a 28 mesh screen (Tylerscale). While aggregates comprised of cokes and/or low-volatile coalssuch as anthracite may be employed advantageously for many purposes, thepreferred aggregate is calcined petroleum coke. Since calcined petroleumcoke has a low mineral matter content, it will not introduce substantialamounts of contaminants into the bath and metal being produced when usedas aggregate in a mastic for foil sheathing of carbons employed in analuminum production cell. Finely divided coke as obtained from theprecipitators or dust collectors of aluminum industry carbon plants isalso a useful aggregate for this purpose.

The proportion of binder tar or pitch mixed with aggregate may beadjusted within the aforesaid weight ranges, and the most suitableproportion may readily be determined, with the particular componentsemployed, so

as to obtain high bond strength and optimum spreadability at atemperature above the softening point of the bituminous material.

Since the mastic composition as described above will be inherentlysomewhat stiff (even at somewhat elevated temperature), it may beconveniently cut back with about 1 to 10 percent, preferably about 3 to5 percent, of an organic solvent for the binder tar or pitch. Aromatichydrocarbons such as high flash coal tar :naphtha and xylenes may beused for brush, spray or roller coating applications, and preferablyhydrocarbon solvents having boiling points of about 130 to 140 C. areused permitting convenient application of the solvent-diluted masticcomposition at a temperature above the softening point of the binder taror pitch and below the boiling point of the solvent.

The mastic composition is most effective as an adhesive between aluminumand baked carbon when applied in a coating weight of about 0.3'to 1.0gram per square inch of surface (solvent-free weight basis). 7

It has been found in accordance with the present invention that bakedcarbon anodes may be afforded effective resistance to air burning,during operation of an aluminum reduction cell, by means of asubstantially airtight sheath comprised of aluminum foil tightly andintimately bonded to the surfaces of the anode by an airexcludingadhesive stratum of a mastic composition as described herein. The foilsheath is conveniently made to extend over the bonnet or top surface ofthe anode and down along the upper side surfaces (preferably for apredetermined distance down the side so that the foil sheath covers onlythat portion of the anode that is to be initially exposed to theatmosphere). The foil sheath may be applied in sections, or moredesirably as a single, unitary sheet, and any overlapping margins bondedin place. Aluminum foil as used herein refers to aluminum in sheet formup to about 0.006 inch thick, and foil about 0.001 to 0.006 inch thickis preferred.

It is important in effecting a reduction in air burning that thealuminum foil sheath be in intimate contact with the surfaces of theanode; otherwise air may diffuse under the metal sheath in the channelsbetween grains of carbon and cause burning. To insure a tight sheath,impervious to air, aluminum foil, preferably in an annealed or softtemper, is bonded to the anode surface by the mastic composition. Foilin a hard temper has a tendency to crinkle or crease, and consequentlymay not as readily conform in an airtight manner to the anode surfaces.Preferably the mastic is applied to the foil surface, and the foil thenapplied to the anode. Alternatively, the anode surfaces may be coatedwith the mastic, both anode and foil being hot, and the foil thenapplied. The mastic will resist the high temperature conditions createdduring operation of the electroyltic cell, so as to retain a goodairtight bond. In addition, the aluminum foil and mastic should notcontain elements or compounds which are detrimental to cell operationsor which may cause undesirable or excessive contamination of the anodecarbon, the electrolyte, or the aluminum metal produced. The masticmeets these conditions, and the aluminum foil recommended in practice ofour invention should be of relatively high purity,

preferably of not less than 99.45% purity. The aluminum foil sheathshould have a thickness of not less than about 0.001 inch. A sheathoflesser thickness may be easily torn or ripped, either duringapplication of the foil to the anode or during installation of the anodein the cell or in operation thereof. Employing a sheath of greaterthickness than 0.006 inch is neither necessary nor desirable, in that asubstantially thicker sheath is required to attain any furthernoticeable improvement against air burning while use thereof may onlyresult in an excessive and uneconomical use of the metal, recovery ofwhich may be small. Aluminum foil-paper laminates may also be employedas sheathing materials where desired.

It will be observed that the aluminum foil sheath obviates the need formaintaining an alumina blanket over the anode. Consequently, with areduced alumina blanket or substantially none at all, a substantialamount of heat produced during operation of the cell will be radiatedfrom the sheathed anode and dissipated into the surround-1 ingatmosphere. To maintain the bath temperature, additional current inputto the cell may be employed, and therefore the amount of aluminum metalproduced per day is increased Without modifying an existing cell designor installed facilities.

The heavy alumina blanket required to protect the anodes from airburning according to conventional practice results in an accumulation offrozen electrolyte and aluminabetween the anodes and sides of the cell,commonly referred to as high backs. A reduced alumina blanket thereforeeliminates high backs from the cell, thus making installation or settingof the anodes in the cell considerably easier. The invention furtherrenders it economical to employ taller anodes in the reduction cell, forexample anodes 23 inches in height as compared to the more normal 18inch anode. Equally important,

the taller anodes may be utilized without altering conventional existingfacilities.

For a better understanding of our invention reference 7 is made hereinto the accompanying figures where FIG- URE 1 is a perspective view of atypical baked carbon anode having an aluminum foil sheath to protect itsupper surfaces against severe air burning; FIGURE 2 is a fragmentarycross-sectional view taken on line 22 of FIGURE 1. The thickness of thefoil sheath and mastic coating is somewhat exaggerated to illustrate themanner of adhesively bonding the sheath to the anode surface.

The invention may further be illustrated by considering the followingdetailed practices fortypical baked carbon anodes for use in aluminumsmelting cells. It was found desirable, when'using mastic cut back withsolvent, to have the anodes at a temperature below 100 C. Mastic coatingweights of about 0.375, 0.5, 0.75 and 1.0 gram per square inch ofsurface were evaluated. In one series of tests the mastic compositionwas 44 percent by weight coal tar pitch having an C. softening point,and 56 percent 28 mesh carbon plant collector dust. This was suitablefor application at 40 C. by spreading. In

another series 44 percent coal tar pitch having a 40 C. softening pointwas used and 56 percent 200 mesh calcined petroleum coke, cut back withabout 3% highflash coal tar solvent naptha (140 C. boiling point), wasapplied at about 130 C. by spraying, Generally, 1l450 plain foil 0.002inch thick was used for sheathing, being applied manually and pressedonto the baked carbon surface with a hand roller. In all of these casesthe foil adhered well to the anode carbon at room temperature and atelevated temperatures (such as 550 C., conveniently used as anevaluation temperature), and in operating tests resulted in reduced airburning and increased aluminum production which the reduced aluminablankets thus made possible.

Having thus described my invention, I claim:

1. In a baked carbon anode adapted for use in an electrolytic cell forthe production of aluminum from alumina dissolved in a moltenelectrolyte and having over its bonnet and upper side surfaces, asubstantially airtight sheath comprised of aluminum foil 0.001 to 0.006inch in thickness, bonded to the aforesaid surfaces by an adhesive, theimprovement in said anode which consists in said aluminum foil sheathbeing tightly and intimately bonded to said baked carbon anode surfacesby a mastic composition consisting essentially of I about 40 to 50percent by weight bituminous material having a softening point less thanabout 110 C. and selected from the group consisting of tar, pitch andmixtures thereof, and about 60 to 50 percent by Weight aggregateselected from the group consisting of low volatile coal, coke andmixtures thereof, the bulk of which passes a 100 mesh screen andsubstantially all of which passes a 28 mesh screen (Tyler scale),

said composition being present in an amount of about 0.3 to 1.0 gram persquare inch of said sheathed surfaces,

said anode being characterized in use by being so protected by saidsheath against air burning as to obviate the need for the conventionalalumina blanket over its upper side surfaces, without introduction intothe cell of materials contaminating the aluminum produced.

2. In a baked carbon anode as set forth in claim 1, the furtherimprovement in which said bituminous material is coal tar pitch having asoftening point of about 40 C. and the amount thereof is about 42 to 47percent by weight, and said aggregate is calcined petroleum coke thebulk of which passes a 200 mesh screen, and the amount thereof is about58 to 53 percent by weight.

3. A carbon electrode having a metal foil sheath adhered to its surfaceby a mastic composition consisting essentially of about 40 to 50 percentby weight bituminous material having a softening point less than about110 C. and selected from the group consisting of tar, pitch and mixturesthereof, and about 60 to 50 percent by weight aggregate selected fromthe group consisting of low volatile coal, coke and mixtures thereof,the bulk of which passes a mesh screen and substantially all of whichpasses a 28 mesh screen (Tyler scale),

said composition being spreadable as a mastic at a temperature above thesoftening point of said bituminous material.

4. A carbon electrode adapted to be exposed to elevated temperaturesapproaching the melting point of aluminum and having a substantiallyairtight sheath comprised of aluminum foil about 0.001 to 0.006 inch inthickness bonded tightly and intimately to its surface by a masticcomposition consisting essentially of about 40 to 50 percent by weightbituminous material having a softening point less than about C. andselected from the group consisting of tar, pitch and mixtures thereof,and about 60 to 50 percent by weight aggregate selected from the groupconsisting of low volatile coal, coke and mixtures thereof, the bulk ofwhich passes a 100 mesh screen and substantially all of which passes a28 mesh screen (Tyler scale),

said composition being spreadable as a mastic at a temperature above thesoftening point of said bituminous material. 5. A carbon electrode asset forth in claim 4 wherein said bituminous material is coal tar pitchhaving a softening point of about 40 C. and the amount thereof is about42 to 47 percent by weight, and

said aggregate is calcined petroleum coke the bulk of which passes a 200mesh screen, and the amount thereof is about 58 to 53 percent by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,500,208 3/1950Shea 106-284 2,683,107 7/1954 Juel 106--284 2,890,128 6/1959 Bushong etal. 106-284 3,060,115 10/1962 Haupin et al. 204290 JOHN H. MACK, PrimaryExaminer.

D. JORDAN, Assistant Examiner.

1. IN A BAKED CARBON ANODE ADAPTED FOR USE IN AN ELECTROLYTIC CELL FORTHE PRODUCTION OF ALUMINUM FROM ALUMINA DISSOLVED IN A MOLTEN ELECROLYTEAND HAVING OVER ITS BONNET AND UPPER SIDE SURFACES, A SUBSTANTIALLYAIRTIGHT SHEATH COMPRISED OF ALUMINUM FOIL 0.001 TO 0.006 INCH INTHICKNESS, BONDED TO THE AFORESAID SURFACES BY AN ADHESIVE, THEIMPROVEMENT IN SAID ANODE WHICH CONSISTS IN SAID ALUMINUM FOIL SHEATHBEING TIGHTLY AND INTIMATELY BONDED TO SAID BAKED CARBON ANODE SURFACESBY A MASTIC COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 40 TO 50 PERCENTBY WEIGHT BITUMINOUS MATERIAL HAVING A SOFTENING POINT LESS THAN ABOUT110*C. AND SELECTED FROM THE GROUP CONSISTING OF TAR, PITCH AND MIXTURESTHEREOF, AND ABOUT 60 TO 50 PERCENT BY WEIGHT AGGREGATE SELECTED FROMTHE GROUP CONSISTING OF LOW VOLATILE COAL, COKE AND MIXTURES THEREOF,THE BULK OF WHICH PASSES A 100 MESH SCREEN AND SUBSTANTIALLY ALL OFWHICH PASSES A 28 MESH SCREEN (TYLER SCALE), SAID COMPOSITION BEINGPRESENT IN AN AMOUNT OF ABOUT 0.3 TO 1.0 GRAM PER SQUARE INCH OF SAIDSHEATHED SURFACES, SAID ANODE BEING CHARACTERIZED IN USE BY BEING SOPROTECTED BY SAID SHEATH AGAINST AIR BURNING AS TO OBVIATE THE NEED FORTHE CONVENTIONAL ALUMINA BLANKET OVER ITS UPPER SIDE SURFACES, WITHOUTINTRODUCTION INTO THE CELL OF MATERIALS CONTAMINATING THE ALUMINUMPRODUCED.